Abstract This paper concerns mainly the area east and southeast of Conception Bay and three islands in the eastern part of that bay, but reference also is made to the Bay de Verde Peninsula west of Conception Bay. Two north-striking fault zones, east and west of the bay, permit distinction of three major fault blocks--an Eastern, a Middle, and a Western block. The rocks of the area are of late Precambrian to early Ordovician age.

The oldest Precambrian rocks cropping out in the area belong to the Harbour Main Group, a sequence mainly of basic and acidic volcanic rocks. These rocks were deformed and then intruded by the Holyrood plutonic series, a large body of mainly granitic rocks. The Harbour Main and Holyrood rocks are unconformably overlain in the Western block by the Conception Group and in the Middle block by the probably equivalent Black Hill sequence. These units consist mainly of fine-grained clastic marine sediments, same of which were silicified during their early diagenesis. A rock sequence of Conception-type lithology also is present in the Eastern block, but it corresponds in age to both the Harbour Main and Conception Groups of the type areas in the Western block. The Conception Group in the wes grades upward into the clastic Hodgewater Group, and the Conception-type rocks in the east grade upward into the Cabot Group, a clastic sequence very similar to the Hodgewater Group. These youngest Precambrian groups seem to have formed in deltaic environments; shaly bottomset beds grade upward into sandy foreset beds, which are overlain by sandy and pebbly topset beds.

The Precambrian ended with deformation, uplift, and erosion. Upon a surface of major unconformity, a marine Cambrian-Ordovician sequence was deposited. At the base of the marine sequence are red and green mudstone and limestone beds (Adeyton Group, Lower to Middle Cambrian), which are succeeded by black, micaceous shale beds and irregularly alternate sandy strata; in the upper part, beds of iron oolite are present also (Harcourt Group, Middle to Upper Cambrian; Bell Island and Wabana Groups, lower Ordovician). In the Western block this sequence has been folded, but in the Middle block it has only been tilted gently.